As you should be aware, flight plans are compulsory when flying in controlled airspace, with of course a few exceptions. It is encouraged that you file one for all your flights, regardless of how long or short, for the simple reason that it reduces your workload, safety between aircraft is improved, ATC are better able to co-ordinate traffic and search and rescue can locate you far quicker should there be an emergency. The purpose here is not to highlight the importance of filing a flight plan (you should), but rather to answer the important question of what to do if circumstances in flight cause you to deviate from the flight plan?
The plan is always to follow the plan, but what happens if you suddenly encounter a menacing thunderstorm in your path, or you get caught in a major updraft? At what point should you contact Air Traffic Services (ATSU) to tell them of the deviation?
Firstly, you need to remember ATSU is concerned with knowing where you are, what your destination is and what time you will be there. If this differs significantly from what is on your flight plan at any point, then you need to talk. So what constitutes a significant deviation?
True airspeed (TAS) – any deviation exceeding 5% of the cruising TAS marked on your flight plan requires you to call the ATSU.
ETA – If the estimated time at the next applicable reporting point, flight information regional boundary, or aerodrome of intended landing, whichever comes first, is found to be in error in excess of two minutes from that on your flight plan, you need to provide a revised estimated time to the ATSU as soon as possible.
In the case of being off track (for example, to avoid a thunderstorm), or a deviation in altitude you should take action as soon as possible to regain the correct track or altitude. It is not necessary to call the ATSU unless your speed or arrival time over a reporting point is affected.